1. Field of the Invention
The present invention relates to a method of repairing and writing a reticle or photomask or any light transmittable substrate. More particularly, it relates to a method of using short duration laser ablation to repair bump and divot defects on a light transmittable substrate.
2. Description of Related Art
The use of lithographic technology in transferring patterns from photomasks to semiconductor substrates in the fabrication of semiconductor devices, such as integrated circuits, has been highly developed and widely used. Reticles, or photomasks, are used with a variety of radiation sources, both visible and ultraviolet, as well as x-rays and electron beams.
Typically, the substrates of a reticle or photomask comprise a transparent material which allows light to pass through preferably at greater than 98% transmittance. Suitable substrate materials include soda lime glass, quartz (lightly doped and undoped), and sapphire. A metal or other light blocking or opaque material is then blanket deposited on the transparent substrate to be patterned and etched to a desired integrated circuit pattern. One type of photomask, a phase shift mask, which may or may not include an opaque material deposited thereon, utilizes etched portions of the quartz substrate itself to form the pattern. Wherein the phase shift mask has a patterned metal layer thereover, etching the quartz also provides a means for attenuating the pattern at the edges to provide higher resolution during photolithography.
In order to prepare the quartz substrate for fabrication of the photomask, the substrate is first planarized to provide a smooth surface within acceptable process parameters. However, planarization according to known means in the art such as plasma etching or chemical mechanical polishing will still leave defects. Generally, bump defects occur when the planarization process does not provide sufficient planarization in a localized area on the substrate surface. Bump defects, if left uncorrected on the substrate surface, alter the phase shift of the substrate. It may also adversely affect the quality of the pattern when the metal layer is deposited thereover since the metal deposited over the bump defect may be semi-transparent. Substrates with bump defects must be scrapped leading to waste and high manufacturing costs. Thus, it would be desirable to provide a means of localized repair for the quartz substrate, and in general, for any transparent substrate.
After planarization of the substrate, the metal or light blocking material is blanket deposited, patterned and etched to the desired integrated circuit pattern. Typically, a photoresist is used to pattern the light blocking material. Etching by means of chemical etching may result in divot defects when over-etching occurs and a portion of the substrate is also removed. Divot defects would alter the phase shift of the photomask reducing the clarity of the projected integrated circuit pattern onto a semiconductor wafer. Given the tight constraints on line width in the current generation and the next generation of semiconductor devices, divot and other defects on the photomasks cannot be tolerated in photolithographic techniques.
The prior art addresses the repair of opaque and clear defects resulting from either too much metal on the substrate, e.g., chrome bridges, or too little metal on the substrate, e.g., pinholes, but does not address repairing the transparent substrate. U.S. Pat. No. 5,035,787 to Parker discloses a method of repairing opaque and clear defects using a focused ion beam (FIB). Opaque defects are sputter etched with minimum mask damage while clear defects are filled in directly from the beam using a source which is compatible with the mask material. However, no mention is made of repairing the substrate, only defects caused by the patterning and etching of the metal layer is addressed.
U.S. Pat. No. 5,382,484 to Hosono describes a method of removing a bump defect in a phase shift mask by coating the bump with a correction film formed by a deposition gas and the FIB, and etching back the film and bump with a focused ion beam using a gallium source. Divots are filled with a spin-on-glass film and etched back with an FIB. Gallium stains and residue from the correction are then removed with an Nd:YAG laser having a wavelength of 532 nm. This technique, although addressing the need for repair of defects on the transparent substrate, utilizes sputter etching with the FIB which can lead to damaged areas printing onto wafers in some circumstances.
U.S. Pat. No. 5,405,721 to Pierrat repairs quartz with a gallium source FIB and removing the gallium stains with wet etching. The problem lies in being able to etch the same depth from the substrate, the top shifting layer and the bottom shifting layer of the phase shift mask to compensate for the phase shift.
U.S. Pat. No. 5,439,763 to Shimase et al. corrects defects in the phase shifter portions of photomasks by using an etch stop layer under the phase shifter comprising Al2O3 during FIB planarization using a reactive gas to planarize the defect down to the etch stop layer. This method requires that an additional layer be added to the photomask which may effect the transmittance of the photomask.
U.S. Pat. No. 5,882,823 to Neary, assigned to the assignee of the present invention, discloses a two-step FIB repair process wherein the FIB is used to core out a quartz bump defect leaving a thin wall of quartz which is removed using an isotropic hydrofluoric acid etch. FIB at an angle may also be used to remove the remaining walls and for removing attached metal defects. However, the wet etch is performed over the entire photomask and is not conducive to localized repairs.
Thus, there remains a need for localized repair of the transparent, particularly quartz, substrate in the fabrication of reticles and photomasks.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of localized repair of the transparent substrate used in reticles and photomasks which would correct bump and divot defects.
It is another object of the present invention to provide a method of localized repair of quartz substrates used in reticles and photomasks.
A further object of the invention is to provide a method of repairing bump defects on transparent substrates used in reticles and photomasks.
It is yet another object of the present invention to provide a method of repairing divot defects on transparent substrates used in reticles and photomasks.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of correcting a bump defect on a phase shift mask comprising the steps of: implanting a light absorbing material into the bump defect; and irradiating the light absorbing material implanted into the bump defect until substantially all of the light absorbing material is removed.
Preferably, the step of implanting a light absorbing material comprises implanting a light absorbing material selected from the group consisting of gallium, boron, phosphorus, arsenic, antimony and combinations thereof. Most preferably, the step of implanting a light absorbing material comprises implanting gallium ions into the bump defect.
Preferably, the step of irradiating the light absorbing material comprises irradiating the light absorbing material with a laser operating at femtosecond and/or attosecond pulses.
In a second aspect, the present invention is directed to a method of correcting a defect in a substrate comprising the steps of: providing a light transmittable substrate, the substrate having one or more bump defects; implanting light absorbing ions into a bump defect on the substrate to form a stained bump; laser ablating the stained bump leaving a corrected surface of the substrate which is substantially planar.
The step of providing a light transmittable substrate may comprise providing a substrate comprising glass, soda lime glass, quartz or sapphire. The step of providing a light transmittable substrate may comprise silicon dioxide doped with about 100 parts per million fluorine. The step of providing a light transmittable substrate may comprise providing a light transmittable substrate having a layer of a patterned light blocking material thereover. Furthermore, the step of providing a light transmittable substrate may also comprise providing a reticle for photolithography.
Preferably, the step of implanting light absorbing ions comprises implanting light absorbing ions selected from the group consisting of gallium, boron, phosphorus, arsenic, antimony and combinations thereof. The list of light absorbing materials is not exhaustive and other light absorbing materials are contemplated. The step of implanting light absorbing ions comprises implanting light absorbing ions such that a concentration gradient of the light absorbing ions extends into the substrate from a top surface of the bump defect. Preferably, the step of implanting light absorbing ions comprises: implanting light absorbing ions into the bump defect such that the light absorbing ions are implanted into the bump defect at a distance equal to a maximum height of the bump defect; and implanting light absorbing ions into the bump defect such that the light absorbing ions are implanted into the bump defect at a distance less than a maximum height of the bump defect such that during the step of laser ablating the stained bump, only those areas stained by implantation are removed to control formation of a substantially planar corrected surface. Preferably, the step of implanting light absorbing ions utilizes a focused ion beam having one or more sources selected from the group consisting of gallium, boron, phosphorus, arsenic, antimony and combinations thereof. Unexpectedly, the step of implanting light absorbing ions controls the depth of substrate material removed during the step of laser ablating the stained bump. Preferably, the step of laser ablating comprises irradiating the stained bump with a laser at such an energy and for a period of time that heat does not dissipate into the substrate utilizing pulses not greater than 10xe2x88x9215 seconds.
In a third aspect, the present invention is directed to a method of repairing a defect on a substrate comprising the steps of: providing a transmittable substrate having thereover a patterned light blocking layer, the substrate having one or more divots thereon; implanting a light absorbing material into the substrate surrounding the one or more divots forming a stained area around the divot; and laser ablating the stained area leaving a corrected surface planar with a surface of the divot utilizing pulses not greater than 10xe2x88x9215 seconds.
During the step of laser ablating the stained area, if a depth of substrate material removed is greater than 60xc2x0 of phase, the method further includes the step of implanting a light absorbing material into non-corrected areas of the substrate to cause a phase of the substrate to be 360xc2x0, and laser ablating the non-corrected surface of the substrate such that a transmission phase for the corrected surface is about equal to a transmission number and phase of the non-corrected surface of the substrate.
In a fourth aspect, the present invention is directed to a method of forming a photomask for photolithography comprising the steps of: providing a light transmittable substrate; planarizing the light transmittable substrate; correcting any defects on the substrate by implanting a light absorbing material into the defects to form a stained defect, and irradiating the stained defect to effectively remove the defect; forming a light blocking layer on the substrate; and patterning the light blocking layer.
The step of providing a light transmittable substrate may comprise providing a light transmittable substrate comprising glass, soda lime glass, quartz or sapphire. The step of correcting any defects on the substrate may occur prior to the step of forming a light blocking layer on the substrate. Alternatively, the step of correcting any defects on the substrate may occur after the step of forming a light blocking layer on the substrate.
Preferably, the step of correcting any defects on the substrate comprises correcting bump defects on the substrate by implanting light absorbing ions selected from the group consisting of gallium, boron, phosphorus, arsenic, antimony, and combinations thereof, into the defects to such a depth that upon irradiating the stained defect, only portions of the substrate which are stained are effectively removed. Preferably, the step of correcting any defects on the substrate comprises correcting a divot defect on the substrate by implanting a light absorbing material surrounding the divot to form a stained defect, irradiating the stained defect, and planarizing a top surface of the substrate to remove additional substrate material equal to a depth of material removed when irradiating the stained defect to retain a prior phase shift of the photomask.
Preferably, the step of forming a light blocking layer comprises depositing a light blocking layer comprising chromium on the substrate. The step of forming a light blocking layer may also comprise blanket ion implantation of a light blocking material into a surface of the substrate.
Preferably, the step of patterning the light blocking layer comprises applying a photoresist over the light blocking layer, exposing a pattern on the photoresist and removing the photoresist.
In a fifth aspect, the present invention is directed to a method of forming a reticle for photolithography comprising the steps of: providing a light transmittable substrate; blanket implanting ions of a light blocking material into a surface of the substrate forming a stained substrate; exposing the stained substrate to laser ablation at pulses of not greater than about 10xe2x88x9215 seconds; and removing portions of the stained substrate in a pattern corresponding to an integrated circuit.
Preferably, the step of blanket implanting ions of a light blocking material comprises blanket implanting a light blocking material selected from the group consisting of gallium, boron, phosphorus, arsenic, antimony and combinations thereof. During the step of blanket implanting ions of a light blocking material, a depth of the ions of light blocking material implanted in the substrate determines a removal depth of portions of the stained substrate. The step of exposing the stained substrate to laser ablation at pulses of not greater than about 10xe2x88x9215 seconds may comprise exposing the stained substrate to femtosecond or attosecond laser ablation for a sufficient number of pulses to vaporize portions of the stained substrate corresponding to an integrated circuit pattern while a radiant energy of the femtosecond ablation does not dissipate into the substrate.